The present invention relates to an image processing apparatus used for projection television or the like.
Various types of image projection apparatuses are used for projection television or the like, with the type of the image projection apparatus adopted being decided according to the intended use. Among them, the system in which the projected image is formed by a one-plane spatial optical modulator uses less elements and can be realized at a lower cost than the system in which three planes of spatial optical modulators are used for the respective ones of the primary colors. For this reason, they are widely used. When a multi-color image is projected by the use of a one-plane spatial optical modulator, projection images of the three primary colors of red, green and blue are formed by the method of time division or spatial division, and any desired color is projected by additive mixing. The method in which the primary color light components are projected by the time division can realize image projection without altering the number of pixels of the spatial optical modulator, and is therefore advantageous in projecting television signals which require a high definition, such as the television signals according to the HDTV (high-definition television) standard. In one method in which the light is time-divided, white light from a light source is passed through a color wheel to generate time-divided primary color light components, which illuminate a spatial optical modulator, to generate images of respective colors.
In a method in which the three primary color light components are time-divided and used for illumination, the light components other than the primary color light component which is being used are not utilized (are reflected or absorbed to cause a loss), so that the light utilization efficiency is low. Improvements in this respect have been desired.
FIG. 8 shows the configuration of a conventional image projection apparatus. Reference numeral 1 denotes a white light source, which includes a lamp 2 and a reflector 3. Reference numeral 4 denotes a first optical means for converging the light emitted by the light source 1. Reference numeral 5 denotes a color wheel formed of color filters of three primary colors. Reference numeral 6 denotes an axis of rotation of the color wheel. Reference numeral 7 denotes a second optical means for converting the light having passed through the color wheel 5 into light which illuminates the spatial optical modulator 8. Reference mark Sd denotes a signal for driving the spatial optical modulator 8. Reference mark L1 denotes light emitted from the light source 1. Reference mark L2 denotes light incident on the color wheel 5. Reference mark L3 denotes light reflected from the color wheel 5. Reference mark L4 denotes light having passed through the color wheel 5. Reference mark L5 denotes light illuminating the spatial optical modulator. Reference mark L6 denotes light having been modulated by the spatial optical modulator. The light L6 is incident on a projection lens (not shown), and is projected on an object (not shown). The object may be a projection screen, photosensitive film, or the like.
FIG. 9 and FIG. 10 show the configuration of the color wheel in a conventional image projection apparatus shown for example in Japanese Patent Kokai Publication No. H5-273673. FIG. 9 shows a color wheel, and reference numerals 21, 22 and 23 denote color filters passing red light, green light and blue light, respectively. The angle occupied by each of the color filters 21, 22 and 23 is 120 degrees. FIG. 10 shows a color wheel including a transparent plate provided in addition to the color filters of the three primary colors, for the purpose of increasing the brightness of the illuminating light. Reference numerals 24, 25 and 26 denote color filters for passing the red light, green light and blue light, respectively. Reference numeral 27 denotes a transparent plate. The angle occupied by each of the filters 24, 25 and 26, and the transparent plate 27 is 90 degrees.
In FIG. 8, the lamp 2 generates light containing red, green and blue light spectra, and the reflector 3 re-directs the light emitted by the lamp 2 toward the first optical means 4. The light L1 is thereby emitted from the light source 1. The first optical means 4 receives the light L1 emitted from the light source 1, and converges the light towards the color wheel. The converged light L2 hits the color filters of the color wheel.
When the color wheel is configured as shown in FIG. 9, and the color wheel rotates 60 revolutions per second, the light L2 is incident at a fixed position, so that the light passing through the color wheel is switched in the sequence of red, green and blue, depending on the rotary positions of the color filters 21, 22 and 23, and this process is repeated 60 times every second. The light having passed through the color filters 21, 22 and 23 is converted by the second optical means 7 into substantially collimated light L5, and illuminates the spatial light modulator 8. The spatial light modulator 8 is driven by the drive signal Sd and modulates the intensity of the light such that the each of the primary colors form a corresponding image, and the light L6 is thereby emitted. The light L6 consists of the light components of the three primary colors emitted sequentially, so that the light components are additively mixed to project images of any desired color. The light components which do not pass through the color filters 21, 22 and 23 are absorbed or reflected, and are not utilized for the light projection. As a result, on average, one third of the light from the light source is utilized for the light projection, while two thirds are not utilized.
When the color wheel is of the configuration shown in FIG. 10, the light L4 repeats changing in the order of red, green, blue and white, 60 times a second. When white light is projected, the luminance is increased, but as the angles occupied by the color filters are reduced, the image becomes dark in the case of highly saturated colors, and the vividness of the colors is lost.
FIG. 11 is a three-dimensional representation of the temporal average of the luminous flux intensity (temporal average luminous flux intensity) of the light L5 illuminating the spatial light modulator 8 in the conventional image projection apparatus. In the figure, reference numerals 31 to 33 respectively denote coordinate axes representing the temporal average luminous flux intensities IR, IG and IB of the primary colors of red, green and blue. Points R1, G1 and B1 respectively represent the temporal average luminous flux intensities of the primary colors of red, green and blue. Point W1 represents the temporal average luminous flux intensity of the light resulting from the combination of the lights of the three primary colors. The inside of a rectangular parallelopiped having its vertexes at the origin O, the points R1, G1, B1, W1, etc., represent the range which can be used for forming projected images by modulation of the intensity of the light L6 by means of the spatial light modulator 8. The larger the rectangular parallelopiped is, the brighter are the images formed by the light projection apparatus, and the wider is the range of expression. The image with a higher luminance can be projected more brightly if the point W1 is farther from the origin O. The image with a higher saturation can be projected more brightly if the points R1, G1 and B1 are farther from the origin.
FIG. 12 is a plan view showing projection of the various points in FIG. 11 onto a plane containing the IG axis and the IB axis. The scales on the axes are arbitrary, but for the purpose of the following comparison, the IG axis coordinate value of the point G1, and the IB axis coordinate value of the point B1 are assumed to be “1.” That is the temporal average luminous flux intensity of each of the three primary colors obtained when the color filters occupying 120 degrees are used is “1.”
FIG. 13 is a plan view showing a projection on a plane defined by the IG axis and IB axis, of the temporal average luminous flux intensity of the light L5 illuminating the spatial optical modulator 8 in the conventional image projection apparatus, in a situation in which the color wheel is of the configuration shown in FIG. 10. The points G2 and B2 represent the temporal average luminous flux intensities of the green and blue primaries, and the point W2 represents the temporal average luminous flux intensity of the light combining the three primary colors. In the color wheel shown in FIG. 10, the angle occupied by each color filter is 90 degrees, so that the IG axis coordinate value of point G2 is 0.75, since 90/120=0.75. Similarly, the IB coordinate axis value of the point B2 is 0.75. The transparent plate passes the light of the three primary colors concurrently, and extends over 90 degrees, so that the luminous flux is increased by 0.75 for each color. For instance, if the white light is added to the light representing the three primary colors, the light intensity is as indicated by W2 in FIG. 13, and this will be the maximum luminance. The range in which the illumination with light is possible extends over the hexagon defined by the origin O, and the points G2, W2 and B2. The ranges on other projected planes are similar. If the coordinate of a point is represented by (IR axis coordinate, IG axis coordinate, IB axis coordinate), the point W2 in FIG. 13 is (1.5, 1.5, 1.5), and is 1.5 times stronger than the coordinate (1, 1, 1) of the point W1 shown in FIG. 12. The coordinate of the point G2 in FIG. 13 is (0, 0.75, 0), so that the green illumination light is 75% of the coordinate (0, 1, 0) of the point G1 in FIG. 12. Accordingly, the image with a high saturation will have the brightness is reduced to 75%.
In the conventional method of increasing the luminance using the color wheel in FIG. 9, it is important that the the proportions between the angles occupied by the color filters and the transparent plate forming the color wheel be so set as to fit the image to be projected. However, it is usually not possible to predict the colors and the saturation of the image projected, so that it is not possible to know the optimum proportion. Moreover, there is a trade-off relation between the luminance of the white peak and saturation of the color. Accordingly, whatever the proportion is determined to be, there are images which will be projected dark.